Okay, let’s get busy. About a year ago, I made stairs to ascend the bank at the back of my yard. I cut the stringers out of four 2×12 10-foot boards, accommodating 9 stairs per stringer, making for 18 saw cuts per board. The 65-minute capture caught the tail end of the first set (board), followed by the next three complete sets.
Four sets of stair stringers (the last three fully captured) were cut with a circular saw, consisting of 18 high-power cuts each.
The time in between cutting periods accounts for measurement and layout work. Measure twice; cut once. The activity shown in the plot represents 370 Wh of work. If the saw peaks at 1800 W, that means the saw was on for an integrated 740 seconds out of the 65 minute span, or 19% of the time. Considering that the cutting phase totals 27 minutes, the actual saw-on time during the cutting phase was about 46%, which seems reasonable. As I cut the wood, I didn’t consider that my efficiency was being quantified by TED. But I happen not to mind spying on myself…
The dominant activity visible here is use of an electric lawn mower, but the sequence starts with laptop activity, the garage door and light, mowing three sections of yard, more garage door action, then back to the laptop.
This sequence shows a variety of actions, starting with laptop use (generally wiggly/variable). Normally our computers are powered by the off-grid solar setup, but sometimes a change of venue might put one of them on utility power. After opening the garage door, I failed to turn the light off manually. Three sections of yard were mowed, costing 57 Wh at a typical power of 600 W. After the mowing was done, the mower was wheeled into the garage, tripping the light—promptly turned off this time—then the garage door was closed and the light turned off immediately after. Naturally, I had to wake up the laptop to export the data from TED.
Our gas furnace uses natural gas at a rate of about 75,000 Btu/hr (22 kW). But the air handling adds 600 W to this. Below are two furnace cycles in our house (rarely used: part of the test described in an earlier post).
Two furnace cycles, showing remarkable consistency. The individual actions are detailed below.
First, the furnace’s internal blower turns on, followed by ignition of the gas about 30 seconds later. After another 30 seconds, the main house blower turns on (accompanied by an initial surge/spike). After about six minutes, the gas turns off, as does the internal blower. The main blower continues for another couple of minutes to deliver residual heat to the house. All told, each cycle is using about 80 Wh of energy (perhaps 240 Wh of fossil thermal energy at a power plant, if operating at 33% efficiency). Meanwhile, the energy expended in gas is about 2400 Wh. So approximately 10% of the expended energy is on the electrical side, or as little as 3% if the source is solar, wind, or hydroelectric—without the typical heat-engine loss.
Another useful measure that comes from such analysis is that, for my house, each Therm of energy (29.3 kWh) delivered by natural gas burned in the furnace is accompanied by 1 kWh of on-site electrical expenditure.